Heat resistant self-piercing rivet for inserting into heated advanced high strength metal

ABSTRACT

A material forming the self-piercing rivet having an elongation of at least about 10%, and the material forming the self-piercing rivet having a hardness of at least about any of 260 HV (Vickers scale), and the material forming the self-piercing rivet being selected from the group consisting of: an age or precipitation hardening steel alloy, a high alloy steel, a stainless steel, and a super-alloy steel. The material forming the self-piercing rivet enables the self-piercing rivet to be set into advanced high-strength metal components, at least one of which is at 400 degrees C. or higher, without cracking

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 62/808,148, filed on Feb, 20, 2019. The entire disclosure of the above application is incorporated herein by reference.

FIELD

The present disclosure relates to self-piercing rivets for insertion into heated advanced high strength metals and using such self-piercing rivets to join pre-heated components.

BACKGROUND

This section provides background information related to the present disclosure which is not necessarily prior art.

As disclosed by U.S. Pat. No. 8,234,770 to Durandet et al. and U.S. Pat. No. 9,815,109 to Savoy et al., lasers have previously been used to preheat metal components that are joined together by setting self-piercing rivets into the pre-heated metal components. The above-identified patents are hereby incorporated herein by reference in their entirety. Other methods of pre-heating the metal components could include induction heating and ultrasonic heating.

DRAWINGS

The drawings herein are for illustrative purposes only.

FIGS. 1-4 provide a cross-sectional view through portions of a self-piercing rivet that has been set into advanced high-strength metal components, at least one of which is at 400 degrees C. or higher; and further illustrates examples of cracks that can form in the self-piercing rivet as a self-piercing rivet deforms in this situation, when attempting to use a self-piercing rivet made of prior art rivet materials.

DETAILED DESCRIPTION

Components made of advanced high strength metals are known to be difficult to join to each other or to other metals. Advanced high strength metals can include advanced high strength aluminums, such as 7000 Series Aluminum. Such advanced high strength aluminums can, in some cases, have a tensile strength of at least any of 500 MPa, 600 MPa, 780 MPa, 880 MPa. Advanced high strength metals can include advanced high strength steels, such as Boron steels, including Usibor 1500, and Usibor 2000. Such high strength steels can, in some cases, have a tensile strength of at least any of 600 MPa, 780 MPa, 980 MPa, 1000 MPa, 1500 MPa, and 2000 MPa, or greater.

During vehicle body assembly for which this invention has particular utility, a sheet metal component of advanced high strength steel or aluminums being joined can, for example, have a thickness of 0.7 to 3.5 millimeters and can be heated, for example, to a temperature of from about 400° C. to about 800° C. In some cases, the temperature can be limited to a temperature that is necessary to avoid adversely affecting the microstructure or other properties of the advanced high strength steel or other metal, such as less than about 730° C.

One problem that can be encountered when setting a self-piercing rivet into heated advanced high strength metal components is as the rivets deform, cracks can appear in the rivets such as shown in FIGS. 1-4.

Such cracking can lead to a weak joint between the two components. For example, the joint between the two components can separate through the rivet. Without intending to be bound by theory, it appears that the cracking is caused—at least in part—by thermal shock to the self-piercing rivet when the unheated rivet is set into the heated metal components. In some cases, this problem can be related to the ductility of the rivet material, which can be related to the elongation of the material. In some cases, the elongation of the rivet material can be at least about any of 10%, 15%, 20%, 25%, 30%, 40%, and 50%.

Another problem that can occur is the rivet legs can collapse when they are set into the heated components. As a result, the rivet does not adequately join the heated components together. In some cases, the hardness of the rivet material can be at least about any of 260 HV (Vickers scale), 300 HV, and 320 HV. In some cases, the hardness of the rivet material can be up to about any of 300 HV, 350 HV, 400 HV, and 420 HV. In some cases, the tensile strength of the rivet material can be at least about any of 500 MPa, 600 MPa, and 620 MPa and, in some cases, can be up to about any of 600 MPa, 700 MPa, and 750 MPa.

Another problem that can occur is that the inherent material properties of the self-piercing rivet can be adversely affected due to the temperature that the rivet is heated to when set into the heated metal components, and then allowed to cool. For example, in a particular heated component joining application the temperature of the heated components can in some cases be at least about any of 400° C., 425° C., 500° C., 600° C., and 700° C., which can be above the transition point for many metals. Thus, the strength of many metals can diminish when subjected to such heating and cooling.

In some cases, it can be desirable to avoid setting a rivet in a heated component at a temperature that is above the temperature to which the rivet material has been heat treated. In other words, it can, in some cases, be desirable to select the rivet material from a steel that has been heat treated to a higher temperature than that which the rivet will encounter when set into a heated component which, in some cases, can be as indicated in the previous paragraph.

Other desirable characteristics of the material of the self-piercing rivet can include corrosion resistance. For example, if the material is inherently sufficiently corrosion resistant, then the need to apply a corrosion resistant coating to the self-piercing rivet after it has been formed can, in some cases, be reduced or eliminated.

In addition, it can be desirable that the material of the self-piercing rivet is such that the self-piercing rivet can be formed from a wire supply and/or by a cold heading process. For example, such a cold heading forming process can increase grain alignment within the material, which can increase the inherent strength of the material and of the rivet. Thus, such a cold heading forming process can, in some cases, reduce or eliminate the need to heat treat the self-piercing rivet after it has been formed.

In some cases, the material that the self-piercing rivet is made of can have a transition temperature that is greater than the temperature to which the high strength metal component is pre-heated, which can be at least any of 425° C., 500° C., or 600° C., or 700° C., or greater.

In some cases, the material that the self-piercing rivet is made of can be an age or precipitation hardening steel alloy. For example, the material can be a precipitation hardening steel alloy.

One category of steels that can be used as the rivet material is high alloy steels. Examples of suitable materials in this category can include the high alloy steel grades selected from SAE 4037, 4140, 4340, and 8620 steels. These and other chromium-molybdenum (chromoly), medium-carbon steels that are heat treatable can be used. Such chromoly steels can be selected that have sufficient strength to avoid compression (rivet legs collapsing) during joint setting, as noted above. Such chromoly steels can be selected that have sufficient chromium and molybdenum to enable these materials to survive brief exposures to the temperatures of the heated components without significant impact to their strength properties. In some cases, the chromoly steel rivet material can have a chromium content of at least about any of 0%, 0.4%, 0.7%, 0.8%, and 0.9%. In some cases, the chromoly steel rivet material can have a chromium content of up to about any of 0.6%, 0.9%. and 1.1%. Additionally or alternatively, in some cases, the chromoly steel rivet material can have a molybdenum content of at least about any of 0%, 0.15%, 0.2% and 0.25%. In some cases, the chromoly steel rivet material can have a molybdenum content of up to about any of 0.25%, 0.3%, and 0.35%. Of course, such chromoly steels can be selected that have any of the other characteristics detailed herein. For example, in some cases, such chromoly steels can have any of a tensile strength from about 500 MPa to about 600 MPa, a hardness from about 320 HV to about 400 HV, and an elongation from about 20% or 22% to about 32% or 35%.

Another category of steels that can be used as the rivet material is stainless steels. Examples of suitable materials in this category can include the stainless steel grades selected from 410, 416, and 420 stainless steels. These and other martensitic stainless steels can be used. Such stainless steels can be selected that include sufficient iron to allow them to be heat treated to or above the temperatures encountered in a particular heated component joining application. In some cases, the iron content of such stainless steel rivet material can be at least about any of 0.03%, 0.08% and 0.15%. In some cases, the iron content of such stainless steel rivet material can be up to about 0.2%. In addition, such stainless steels can be selected that have sufficient ductility to prevent cracking in a particular heated component joining application, as noted above. Such stainless steels can also be selected that have sufficient chromium to provide the desired thermal stability at the temperatures encountered in a particular heated component joining application. In some cases, such stainless steel rivet material can have a chromium content of at least about any of 10%, 11%, and 12%. In some cases, such stainless steel rivet material can have a chromium content of up to about any of 13%, 14%, and 15%. Of course, such stainless steels can be selected that have any of the other characteristics detailed herein. For example, in some cases, such stainless steels can have any of a tensile strength from about 500 MPa to about 600 MPa; a hardness from about 320 HV to about 400 HV; and an elongation from about 10% or 15% to about 30% or 35%.

Another category of steels that can be used as the rivet material is super-alloy steels, such as those typically used for aircraft components. Examples of suitable materials in this category can include the super-alloy steel grades selected from A-286, Inconel 718, Hastelloy, and Nimonic 80A. These and other super-alloy steels can be used. Such super-alloy steels can be selected that have sufficiently high nickel and/or chromium to make them extremely heat resistant at the temperatures encountered in a particular heated component joining application. In some cases, such super-alloy steel rivet material can have a chromium content of at least about any of 13%, 14%, 15%, 16% 17%, and 18%. In some cases, such super-alloy steel rivet material can have a chromium content of up to about any of 15%, 16%, 18%, 20%, and 21%. Additionally or alternatively, in some cases, such super-alloy steel rivet material can have a nickel content of at least about any of 22%, 48%, 58%, and 68%. In some cases, such super-alloy steel rivet material can have a nickel content of up to about any of 28%, 58%, 62%, and 71%. Such super-alloy steels can be selected that gain strength during heat exposure, which is sometimes referred to as age hardening or precipitation hardening. Of course, such super-alloy steels can be selected that have any of the other characteristics detailed herein. For example, in some cases, such super-alloy steels can have any of a tensile strength from about 600 MPa, 620 MPa or 700 MPa to about 700 MPa, 820 MPa, or 1,250 MPa; and a hardness from about 250 HV or 275 HV to about 300 HV or 350 HV; and an elongation from about 10% or 15% to about 30% or 65%.

Of course, the use of self-piercing rivets made of the rivet materials described herein, for example, in processes such as those described in U.S. Pat. No. 8,234,770 to Durandet et al. and U.S. Pat. No. 9,815,109 to Savoy et al., as well as other joining component heating processes is contemplated herein.

The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure. 

What is claimed is:
 1. A self-piercing rivet for setting into advanced high-strength metal components, at least one of which is at 400 degrees C. or higher, the self-piercing rivet comprising: a material forming the self-piercing rivet having an elongation of at least about 10%, and the material forming the self-piercing rivet having a hardness of at least about any of 260 HV (Vickers scale), and the material forming the self-piercing rivet being selected from the group consisting of: an age or precipitation hardening steel alloy, a high alloy steel, a stainless steel, and a super-alloy steel; wherein the material forming the self-piercing rivet enables the self-piercing rivet to be set into advanced high-strength metal components, at least one of which is at 400 degrees C. or higher, without exhibiting cracking. 